A PON system is a system in which an optical line terminal serving as a central station is connected to optical network units installed in a plurality of subscribers' homes by an optical fiber network where a single optical fiber is split into a plurality of optical fibers through an optical coupler.
In this case, if the plurality of optical network units simultaneously perform transmission to the optical line terminal, then transmitted data units collide with each other. Thus, the optical line terminal provides the optical network units with grants for a transmission timing and an amount of transmission data. Each optical network unit performs upstream transmission of the amount of transmission data allowed by the optical line terminal at the timing allowed by the optical line terminal.
Since the distances between the optical line terminal and the optical network units vary between the locations where the optical network units are installed, the strengths of upstream optical signals received by the optical line terminal are not uniform. Thus, upstream optical signals with various strengths intermittently arrive at the optical line terminal from the plurality of optical network units.
Hence, there is already proposed a technique in which, in order to improve the reception responsivity to an optical burst signal, the response speed of a receiving apparatus to a burst signal is changed between a synchronization section (preamble) and a data section (payload) (see Patent Literature 1).
In this conventional art, specifically, a small time constant is set for the synchronization section to increase the followability for a burst signal, and a large time constant is set for the data section to enhance the reception tolerance to consecutive identical digits.
In addition, there is already proposed a technique in which in order that a pre-amplifier suitable for an optical receiver module of an optical line terminal can stably amplify a broadband burst signal, the operating mode of the amplifier is made switchable to either a gain adjustment mode or a gain fixed mode (see Patent Literature 2).
In this conventional art, specifically, in the adjustment mode, an appropriate gain is determined from among two levels, based on the strength of a burst signal in a synchronization section, and after the determination, the burst signal is amplified in the fixed mode where the gain is fixed.